The effect of compressibility on the pressure reading of a Prandtl pitot tube at subsonic flow velocity Page: 4 of 14
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N.A.O.A. Technical Memorandum No. 917
or less severe disturbance in the flow at the static ori-
fice of t-he Prandtl pitot, with the result -that the pros-
sure of undisturbed flow can no longer be measured at that
Strictly speaking, the speed prediction with the
Prandtl"pitot therefore is contingent upon the knowledge
of the errors of the pressure readings from calibration
of the instrument and their correction. The experinental
determination of this correction is related in the follow-
io tat ion
V w, velocity in undisturbed flow;
c a, velocity of sound in undisturbed flow;
p, density in undisturbed flow;
p, pressure in undisturbed flow;
po, tank pressure;
P ppressure at forward orifice of Prandtl pitot;
4p2, pressure at lateral orifice of Prandtl pitot;
/ K, ratio of specific heat at constant pressure and
constant volume, respectively.
DESCRIPTION OF TEST EQUIPMENT
The high-speed tunnel (Prandtl type) employed is shown
in figure 1. A container B is pumped empty so that the.
atmospheric air (pressure po, density Po, and speed
wo = 0) flows, after opening of cock H, as a free jet
through the test chamber M into the container. On leav-
ing the entrance cone E the air stream attains the speed
w, the pressure expands to p and the density to p; a
indicates the velocity of sound in the free jet. The wind
velocity w is regulated with the nozzle V whose nar-
rowest section is adjustable. So long as the pressure in
B stays low enough to produce velocity of sound in the
narrowest section of V, the jet remains stationary and
the flow volume and jet velocity, respectively, change only
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Walchner, O. The effect of compressibility on the pressure reading of a Prandtl pitot tube at subsonic flow velocity, report, November 1939; (digital.library.unt.edu/ark:/67531/metadc63232/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.